There’s the issue of personality compatibility. A disagreeable partner who conscientiously tries to fix the conflicts they start can still be a constant source of anxiety to someone who doesn’t like confrontation. Another who believes success is most important might find a partner who’s not conscientious to be unworthy of their love and respect, even if they have loads of other positive traits. An extroverted person might find the solitude that’s preferred by their introverted partner to be exasperating.
And even if partner’s personalities are in sync, the conditions of the marriage can upset that balance. Personalities have some fluidity, and environmental conditions can affect how we think and act.
Consequently, an open-minded person may become frustrated and combative if married to a close-minded individual. If a partner is uncommitted or untrustworthy, an an otherwise emotionally stable person might gradually become chronically anxious and angry. An agreeable person may become less agreeable if they have to cope with chronic negative events, such as financial problems.
It’s never easy.